About 3 weeks ago, I noticed that my Disco II would enter open loop at any speed over 35mph when I took my foot off the accelerator. Once the truck goes under 35mph with my foot off the gas, it enters back into closed loop. Any speed under 35 mph, it stays in closed loop (on or off the accelerator). I'm pretty sure this wasn't the case before the past three weeks. The truck also is in open loop when i start it up, and before I put my foot on the accelerator, both on hot and cold start up. It enters closed loop when I start to drive. Is this normal, or indicative of a problem?

Also (and I know I'm going to catch some heat for this) about 2 weeks ago, I decided to try running the Disco on 89 octane, just to see what happened. For the past 3 months, I've been running 93 octane. According to my ultragauge, which I have had hooked up since May, my truck runs at an average of 15.7 mpg, city/highway mix. It has never strayed from 15.7. After running a half tank of 89, my mpg went up to 15.8. After a full tank, it went up to 15.9 mpg. Now, I drive the same routes every week, and put the same amount of weekly mileage on the truck. I also know that my truck runs a little cooler than others (183.8-186.8 highway, 188.6-190.4 city, and 194-195.8 at idle), and I live at sea level. While this isn't indicative of much, I just thought I would throw that out there and see what you guys thought.

 

Your truck is supposed to stay in closed loop after it warms up and should not be going back to closed loop, start up rich mode.
As for the octane, stay with 93 octane, you will get more torque and better mileage and not stand the chance of carboning up your valves.
By the way, never base the scan gauge MPG as accurate, it is a combination guess and average and not accurate.
If you want real mileage, do it the old fashion way and divide the miles by the gallons.

 

If you read the Ultra-Gauge owners manual it explains the open/closed loop in great detail.
Basically it works like this, when cold it runs in open loop, which is full choke, after warm up it switches to closed loop and delivers fuel based on need.
If you start the engine from cold and just sit and watch it will go to closed loop after a few min depending on how cold the temp is and how old the O2's are.
It was 100*F here today, mine went to closed loop right away, in the winter it can take 5+ min to get to closed loop.
As for the take your foot off the gas open loop, that is normal, it will even happen at highway speeds, take your foot off the gas to coast down a exit ramp, open loop, I have no idea why.
As for the fuel rating and MPG, the only way for you to know for 100% if you are getting better MPG with the 89 octane is for someone else to fuel your truck and not tell you what grade of fuel they are using.
Driving the same route means little I have found out, it is more based on your driving habits.
I slowed down from 65mph to 60mph driving back and forth to work and raised my MPG by 2-3 but only added 10 min round trip.
I use a MPG app on my iPhone, the Ultra-Gauge is pretty close to the app which uses the miles/gal method which is the most accurate.
Mileage Keeper is the app, its free.

 

Some fuel injection systems have a fuel injection cut off feature when you let off the accelerator. If these do that, it might consider that open loop because it doesn't care about the sensors, everything is preprogrammed to off at that point.

 

The open loop / closed loop switch happens when exhaust warms up, and for very short periods during normal operation of the vehicle if what you are doing is considered by the ECU to be handled better by relying on stored adaptive values, etc. From the RAVE (various spots):

The heater elements are supplied with current immediately following engine start and are ready for closed loop control within about 20 to 30 seconds (longer at cold ambient temperatures less than

0°C (32°F)) [So this has zip to do with coolant temp warming up, what color the water hose is, etc.]

From cold start, the ECM runs an open loop fuelling strategy. The ECM keeps this strategy in place until the HO

2S is at a working temperature of 350 °C (662 °F).

To achieve optimum fuel quantity under all driving conditions, the ECM provides an adaptive fuel strategy.

Conditions

Adaptive fuel strategy must be maintained under all throttle positions except:

l

Cold starting.

l

Hot starting.

l

Wide open throttle.

l

Acceleration.
All of the throttle positions mentioned above are deemed to be 'open loop'. Open loop fuelling does not rely on information from the HO2 sensors, but the air/ fuel ratio is set directly by the ECM. During cold start conditions the ECM uses ECT information to allow more fuel to be injected into the cylinders to facilitate cold starting. This strategy is maintained until the HO2 sensors are at working temperature and can pass exhaust gas information to the ECM. Because of the specific nature of the other functions e.g. hot starting, idle, wide open throttle, and acceleration they
also require an 'open loop' strategy. For NAS vehicles with secondary air injection for cold start conditions, refer to the Emissions section.

Disco Mike is right that it should quickly enter closed loop when cranked up, and most of the time it should stay there. If it does not, (1) your mpg will stink, (2) you are spending too much on Red Bull (and driving like you stole it)...

 

OK, so open loop on start up (both hot and cold) is normal per Spike's quote from the RAVE. My truck enters closed loop within 10 seconds of hot startup, and I would guesstimate about 30 seconds on cold startup. Can anyone confirm what Jafir said about the ultragauge reading the fuel cutoff as open loop (I believe what you say, Jafir, I just want to know if the ultragauge reads that as open loop)? Like I mentioned above, it enters open loop ONLY at speeds over 35mph when I take my foot off the gas.

Also, I'm a Granny behind the wheel. I never get over 70 mph on the expressway unless I'm going down a hill. Right now, I just filled up the tank. I'm going to calculate the mpg on the 89 I have in there now the old fashion way, and then next week I'll switch back to the 93 and calculate that the same way and see what happens. I just thought it was interesting to see a fluctuation so soon after switching, especially since I've put 6000 miles on the truck since I set up my ultraguage, and it never once moved from 15.7 average mpg until the switch in octane.

 

You don't have to believe it... I'm just thinking out loud. I could certainly be wrong and it won't hurt my feelings.

 

Hahaha. Well, my man, I read your posts. You're a knowledgable guy. I just want to make sure there isn't something off with my truck. The Red Dragon is running so well right now, I'd like to nip any problems in the bud before they explode into something huge, which is why I'm trying to track down why it's entering into open loop so often.

 

He is only a little wrong, there is no switch, the ecu monitors the temp, RPM, throttle pressure, all that crap and it can drop the fuel pressure, but only to the point of bring all the sensor readings back into line everytime you change the throttle pressure.

 

I would think you would have codes if you want open loop in the middle of a trip or such.

This would portend that the O2 sensors are not delivering an acceptable voltage.

This will generate a code eventually.
You may want to check for pending codes.

You may want to check your O2 sensor wires and make sure they are not melting into the cat or the pipes.

I had an O2 sensor that just went open or shorted once and a while and was causing codes.

Replaced it.
Just went on a 1531 mile trip.
No codes.
Used premium gas from all sorts of over the road vendors - Pilot -
no name, Boulder Gas

17.4 MPG.
Using the "Mike" old fashioned tried and true method.
Fill tank.
Miles to "0"
Drive to where you are going.
Fill tank.
Add up receipts
and there is your MPG.

Don't get too carried away with the scanner.
The ECU is scanning for you.
And will give you a CEL - if you like it or not.